I. Field
The present disclosure relates generally to communication, and more specifically to techniques for transmitting data in a communication system.
II. Background
A cellular system is a wireless multiple-access communication system that can concurrently communicate with multiple terminals, e.g., cellular phones. A cellular system may employ space-time coding in order to achieve space and time diversity for a transmission to a terminal. Space-time coding refers to processing of data (e.g., a modulation symbol) such that the data is sent via multiple physical antennas and across time to achieve both space and time diversity. For example, a third generation (3G) cellular system may employ a space-time coding scheme such as space-time transmit diversity (STTD) or orthogonal transmit diversity (OTD) to transmit each pair of symbols from two antennas in two symbol periods. STTD and OTD are different but fixed schemes for mapping two symbols to two antennas in two symbol periods. STTD and OTD generally improve performance over the case in which space-time coding is not employed.
However, STTD and OTD have several major drawbacks. First, STTD and OTD typically suffer performance loss in time dispersive multipath environments, which cause frequency selective fading and loss of orthogonality. Second, STTD and OTD are limited to operation on two transmit antennas. Third, STTD and OTD are fixed space-time codes. In some scenarios, such as for a stationary or slow moving terminal, the wireless channel may be in a state that does not match very well with the fixed space-time code for STTD or OTD and may remain in this bad state for an extended period of time. All of these drawbacks limit the use of and/or hamper the performance of STTI) and OTD.
There is therefore a need in the art for transmission techniques that can provide good performance in multipath environments.